The present disclosure relates generally to systems and tags used for tracking the locations of objects, and is more particularly concerned with an object tracking system in which more than one type of object tracking procedure is employed.
It can be highly advantageous in the environment of a factory to be able to keep track of the current locations of certain objects. For example, it may be desirable to keep track of the locations of tools, production equipment, inventory and/or the items being produced in the factory. Object location tracking is also potentially applicable to other environments, such as warehouses, vehicle parking lots, railroad yards, container terminals, and the like.
FIG. 1 is a schematic plan view representation of a conventional object tracking system 100. The object tracking system 100 is installed in a factory facility 102. The object tracking system 100 utilizes tags 104 (represented in the drawing by octagonal symbols that also serve to represent objects, not separately shown, to which the tags are affixed to permit tracking of the objects).
The object tracking system 100 utilizes two different procedures—proximity detection and triangulation—to track the tags 104. Interrogation gates 106 are used for proximity detection, and triangulation stations 108 allow tag locations to be determined by triangulation. Another significant element of the system 100, but not shown in the drawing, is a central computer that is coupled by signal paths (also not shown) to the interrogation gates 106 and triangulation stations 108.
In accordance with conventional practices, a tag 104 that is in proximity to an interrogation gate 106 receives an interrogation signal from the interrogation gate and responds to the interrogation signal by transmitting a response signal that includes a tag identification code that uniquely identifies the tag. The interrogation gate then effectively reports to the central computer that the particular tag is at the interrogation gate. The interaction between the tag and the interrogation gate may be in accordance with conventional RFID (radio frequency identification) practices. In other variations, the interrogation gate may read a barcode or the like from the tag.
The tags 104 send out signals at brief regular intervals which are received by triangulation station 108. By using the triangulation stations 108, the central computer utilizes a triangulation procedure to determine the location of tags that are not in proximity to one of the interrogation gates 106. More specifically, the central computer may use a differential time of arrival (DTOA) procedure in which a tag ID signal transmitted by a tag 104 is received by three or more of the triangulation stations 108. Differences in the timing at which the tag ID signal is received at each triangulation station are used by the central computer to calculate the location of the tag, based on the locations of the stations 108 which received the tag ID signal. For example, in FIG. 1, a tag ID signal transmitted by tag 104-1 is received by line-of-sight at triangulation stations 108-1, 108-2, 108-3, thereby allowing the location of tag 104-1 to be determined by triangulation. Similarly, a tag ID signal transmitted by tag 104-2 is received by triangulation stations 108-4, 108-5, 108-6 so that the location of tag 104-2 can be determined by triangulation.
The “MOBY R” object locating system available from Siemens A G, an assignee hereof, is an example of a system that employs DTOA to locate objects.
In some examples of a conventional object tracking system, the number of tags may be in the thousands, and the number of interrogation gates and/or triangulation stations may be in the dozens.
An object tracking system as illustrated in FIG. 1 often operates effectively to achieve its intended purposes. However, in some cases such systems may exhibit drawbacks that it would be desirable to address. For example, triangulation by DTOA requires line-of-sight transmission from a tag to three or more triangulation stations and thus works best in open, unobstructed areas. Disadvantageously, some factory environments may have a significant number of obstructions to tag ID signal transmission, such as the obstructions 110 shown in FIG. 1. When obstructions are present, it is usually necessary to provide an increased number of triangulation stations to avoid “dead spots” in which tags cannot be detected by triangulation. This increases the cost of the tracking system. Furthermore, the presence of obstructions increases the amount of time required for planning the system and determining the locations at which triangulation stations are to be installed. This too increases the cost of the system, and also increases the time required to deploy the system.
Moreover, “temporary” obstructions, such as loaded pallets, trucks, forklifts, etc., may interfere with triangulation capabilities of the system. Consider for example the case of tag 104-3 shown in FIG. 1. It is assumed that a temporary obstruction is placed as indicated in phantom at 112, blocking the line-of-sight transmission path from tag 104-3 to triangulation station 108-4. As a result, line-of-sight transmission is possible from tag 104-3 only to two triangulation stations, namely stations 108-7 and 108-8. Consequently, the location of tag 104-3 cannot currently be determined by DTOA.
Even in the absence of such problems, reflections of tag ID signal transmissions may adversely affect performance of the DTOA procedure.
In simpler object tracking systems, only interrogation gates are employed. However, in such systems, the location of an object is known only when it is in proximity to an interrogation gate. If, for example, a gate is provided at the entrance to a large enclosed area (e.g., a warehouse or parking lot), it may be possible to determine that an object is in the enclosed area, but finding the object within that area may be difficult, and is not aided by the object tracking system.